1. Field of the Invention
The present invention generally relates to water fraction meters or moisture content meters, and more particularly to a narrow band infrared water fraction meter for gas well and liquid hydrocarbon flow stream use.
2. Description of the Related Art
Natural gas wells often produce water along with the natural gas during normal production. The water is resident in the reservoir and frequently accompanies the natural gas as it flows up to the surface production equipment. Normally, the fraction of the overall flow stream that is comprised of water is small on a volume percentage basis but this value can change. The entrained water can lead to pipeline corrosion and gas hydrate formation, which can actually plug the pipeline. Operators periodically measure the water fraction of the gas well to insure that the corrosion inhibition and hydrate inhibition schemes (typically chemical injection) are adequately sized. The water fraction measurement is also a indicator of the general health of the well.
Conventional methods for determining a natural gas well's water fraction are either inaccurate or expensive. A common method for gauging the water content as it relates to corrosion is to place test metal coupons in the pipeline for an extended period of time. The coupon is then analyzed for corrosive degradation, and an average corrosion rate is determined. This technique allows for an average water content determination, but to handle real life variations, operators have to design the chemical injection rate of the corrosion inhibitor for the estimated highest instantaneous water fraction. This conservative approach is very inefficient and can result in excess use of inhibition chemicals, which can cost tens of thousands of dollars per well per year.
Another common method of water fraction measurement involves separation of the water and natural gas with relative measurement of each stream. These separators can give better real time data (at least on a daily average basis), but the equipment can be large and prohibitively expensive for individual wells.
Online sensors that measure the relative concentrations of water and gas without requiring separation of the two phases are the preferred method for optimizing chemical injection. While there are a few sensor technologies currently being employed as water fraction or moisture content analyzers, most are either too expensive for individual wellhead application or they are very susceptible to dissolved mineral content in the water phase.
Examples of water fraction or moisture content analyzers are described in Kassen et al., U.S. Pat. No. 5,369,368, Helms et al. U.S. Pat. No. 4,947,129, Mactaggart, U.S. Pat. No. 4,171,918, Saito et al., U.S. Pat. No. 5,870,926, Murray Jr. et al., U.S. Pat. No. 5,107,118, Lew et al., U.S. Pat. No. 4,785,245, Stephenson et al., U.S. Pat. No. 5,689,540, and Mougne, U.S. Pat. No. 5,067,345. Kassen et al. and Helms et al. describe examples of devices which use microwave energy and its measured transmission or reflection through the target media to determine the water content or cut. In both cases, a phase shift indicates a change in water content when compared to a reference condition. Lew et al. describes a water cut meter that employs nuclear magnetic resonance (NMR) analysis. In this device, traditional pulse NMR techniques are used to determine the percentage of one component of a multiphase fluid (oil, water, gas, and soil particles) flowing in a pipeline. Stephenson et al. describes a water fraction meter that uses X-rays. An X-ray generator provides a continuous bremsstrahlung photon spectrum to a pipeline stream containing a mixture of oil, water, and gas. Photons from multiple detectors are measured, and an algorithm is used to determine water cut. Mougne describes an apparatus for calculating bulk water in crude oil or steam using a capacitance measurement. It is an in-line probe capable of measuring "bulk" capacitance with a way to calculate the water content based on the measured signal.
Mactaggart, Saito et al., and Murray Jr. et al. all describe infrared based moisture or water content analyzers. Mactaggart and Saito et al. describe devices for determining the moisture content of a material by measuring the relative reflectance at two infrared wavelengths. Murray Jr. et al. describes an infrared device that measures transmission of infrared energy at a frequency in the 3700-4000 cm.sup.-1 range. A reference measurement is then made with a "dry" sample, and the ratio of the values is an indication of the moisture content.